It is known that modern internal combustion engines (ICE) include after treatment devices, such as diesel oxidation catalysts (DOC) and Diesel particulate filters (DPF), which are disposed in an exhaust pipe of the ICE to change the composition of the exhaust gases, thereby reducing the polluting emissions. Some ICE may also include a selective catalytic reduction (SCR) system and an EGR system, both of which are generally provided for reducing the content of nitrogen oxides. The SCR system usually includes a catalyst disposed in the exhaust pipe of the ICE and an injector of a reducing agent located upstream of the catalyst. In this way, the injected reducing agent is stored inside the catalyst, where it is used to convert nitrogen oxides into diatomic nitrogen and water.
The EGR system usually includes an EGR conduit that fluidly connects the exhaust gas pipe with the intake pipe of the ICE, in order to recirculate part of the exhaust gases back into the combustion chambers. In this way, the recirculated exhaust gases are able to reduce the combustion temperature and thus the nitrogen oxides produced by the ICE. While significantly reducing the quantity of nitrogen oxides, the recirculation of exhaust gas has the side effect of increasing the amount of soot and smoke emitted by the ICE, which is not only harmful for the environment but it may also bring to more frequent regeneration of the particulate filter.
For this reason, the quantity of exhaust gas routed back to the intake pipe is regulated by means of an EGR valve, which is disposed in the EGR conduit and which is connected to an electronic control unit (ECU). More particularly, the electronic control unit (ECU) is configured to regulate the quantity of exhaust gas recirculated by the EGR system by determining a set point of the air mass flow rate necessary to operate the engine, and by controlling the EGR valve in order to reach the set point. The set point of the air mass flow rate is conventionally determined on the basis of several parameters that are linked to the engine cycle (e.g. the engine speed, the injected fuel quantity, the gear, etc.).
However, with these parameters, the quantity of gas recirculated by the EGR system cannot be always adjusted to optimize the overall performance of the ICE (seen as a system that also includes other exhaust treatment devices like the SCR system), in order to reduce the global emission of pollutants. In particular, it is not always possible to adjust the quantity of gas recirculated by the EGR system in order to reach an effective compromise between the quantities of nitrogen oxides released in the environment and the smoke and soot produced by the ICE.